Vacuum tube amplifier circuits



Dec. 22, 1936. WO L 2,065,257

- VACUUM TUBE AMPLIFIER CIRCUITS Filed April 11, 1934 Hal/RE I.

CHHRLfJ fkL'DER/CK WOLL f/VVE/VTOR firm/awe 7-8 Patented Dec. 22, 1936 UNITED STATES PATENT OFFICE VACUUM TUBE ABIPLIFIER CIRCUITS vania Application April 11, 1934, Serial No. 720,064

6 Claims.

My invention relates to vacuum tube amplifier circuits. More specifically, my invention employs regenerative connections which compensate for degenerative effects in an audio frequency amv plifier. Further, my invention makes possible an efi'icient and inexpensive method of regeneration of definite audio frequencies.

The broad principles of regeneration are well known to those skilled in the art. One form of regeneration or feedback in a resistance coupled vacuum tube amplifier has been shown in the Hartley U. S. Patent No. 1,218,650. Another specific feedback circuit has been disclosed in the Brillouin U. S. Patent No. 1,404,574. The arrangements I employ are different from the disclosures mentioned above and others of similar nature, and may be best understood by reference to the accompanying figures and specification.

Figure I represents a vacuum tube amplifier circuit which has a biasing resistance common to input and output circuits.

Figure II represents a specific circuit embodying my invention.

Figure III represents a circuit employing triode tubes in which a reflexed connection from the common resistance of an output tube to the input circuit of a preceding tube is employed.

In Figure I, l represents a three electrode vacuum tube with a cathode 2, grid 3 and plate 4.

The input circuit is grid 3, input resistance 5, bias resistance 6, and cathode 2. The output circuit consists of plate 4, output transformer l, B battery 8, bias resistance 6, and cathode 2. The cathode may be heated by battery 9. The positive pole of B battery 8 is connected to the plate 4.

, The negative pole of the B battery connects to the junction of resistances 5 and 6..

The steady plate current flowing through resistance 6 produces a voltage drop in the resistance which makes the cathode positive with respect to the grid. That is, the grid of the tube is biased negatively by the current flowing in its plate circuit through resistance 6. The current to be amplified is impressed on resistance 5. The electro-motive force in the input circuit is represented by amplified current in the plate circuit. The amplified current in the plate circuit flows through resistance 6 creating a voltage 50 drop which is out of phase with the voltage in the input circuit. This effect decreases the amplification of the tube and under some conditions the apparatus may become a de-amplifier. The circuit is said to be degenerative because the feed- 55 back from the plate circuit to the grid circuit through the common resistance acts exactly opposite to the well known regenerative action which builds up the input voltage.

In practice this effect is very serious at low audio frequencies and is usually overcome by 5 shunting across biasing resistance 6 a large capacity. The size of this capacity is chosen so that its reactance will be extremely low at low audio frequencies. In many cases capacities as high as twenty-five microfarads are required for 10 satisfactory amplification. Condensers of such large capacities are relatively expensive.

Referring to Figure II, I0 represents an antenna system connected in series with primary 1 l and grounded at 12. The primary ll iscoupled to 15 secondary inductance 13 which may be made resonant to an incoming signal current, by variable capacity l4 shunted across the terminals ofv the inductance. One terminal of this resonant circuit is grounded at l2. The opposite terminal is connected to the grid I5 of screen grid detector tube I6 through grid condenser I! which is shunted by grid leak resistance l8. The cathode I9 is grounded at l2. The plate 2|) connects through resistance 2| and capacity 22 to the grid 25 23 of the audio frequency amplifier tube 24. Grid 23 is connected to grid leak resistance 49 which connects to the ungrounded terminal of 45. The plate 20 is also connected to capacity 25 which is grounded. Resistance 2| and capacity 30 25 form a radio frequency filter which prevents the radio frequency currents from entering the audio amplifier. Between 2| and 22 is connected one terminal of a resistance 26; the other terminal of 26 is connected to the positive terminal 35 48 of the plate current supply. The screen grid 21 of tube I6 is biased positively by a suitable resistance 28 which is connected to the positive terminal 48 of the plate current supply.

The cathode 29 of tube 24 is grounded at [2. The plate 30 is connected to ground through capacity 32. The plate 30 is also connected to one terminal of the primary of output transformer 33. The remaining terminal of the primary of 33 is connected to the positive terminal 48 of the plate current supply. The screen grid 34 is connected to 48. The secondary terminals of output transformer are connected to voice coil 35 of dynamic speaker 36.

The plate current supply may be obtained by rectifying and filtering alternating current from any suitable source or direct current may be used in place of a rectifier system. A transformer 31 its primary connected to a 110 volt supply A secondary winding increases the voltage to about 250 volts and is connected to plate 38 of rectifier tube 39 and to the negative lead 42 of a filter system. The cathode 40 of tube 39 connects to a filter choke 4| which may be the field winding of dynamic speaker 36, or an entirely separate choke. Between the terminals of 4| and negative lead 42 are connected two smoothing capacities 53 and 44. It will be understood that the filter 4!, 43, and 44 smooths out the pulsating current supplied by the rectifier, and furnishes a substantially smooth, unvarying direct current source of power. The negative lead 42 isgrounded through resistance 45. This resistance 45 is the biasing resistance for audio amplifying tube 24. A tertiary winding N6 of transformer 31 supplies the current for heating the filaments of tubes I6, 24 and 59.

The operation of the audio frequency amplifier of the circuits thus far described is subject to the degenerative action which results from resistance 45 being common to the grid circuits and plate circuits of tube 24. This degenerative action has been described in connection with Figure I. It must be overcome if efficient amplification is to be obtained from tube 24.

I overcome this deleterious feedback by taking part of the out of phase voltage drop across resistance 35 and applying the voltage to the screen grid 2] of tube l6. The voltage thus fed to 2'! will now reappear as amplified current in resistance 26. The voltage across 26 will be in proper phase to augment the voltage originally applied to the input circuit of tube 24. The connections employed are the two capacities 46 and 41 which are serially connected across resistance 45. The junction of 46 and 41 is connected to the screen 2?.

The ratio of the capacity 46 to the sum of capacities 46 plus i! will determine the amount of voltage fed back to the screen grid 21 of tube [6. If this feedback voltage is made too large, the resultant overall feedback; i. e., the total regeneration minus the total degeneration, will be sufficient to sustain a continuous cyclic effect, which is manifest by 'a singing or sustained disturbance in the audio output. While a theoretical consideration of the principles indicate that the degeneration in tube 24 might be excessive and partially nullify the regenerative feedback, in practice I have not found such effects when employing normal tubes, resistances and voltages of the values recommended by the manufacturers of tubes now commercially available; such as, the R. C. A. 57 and 2A5 tubes. 6

An audio output tube operating a normal output transformer and speaker is generally least efficient at the lower audio frequencies. It is these frequencies which are desired in faithful reproduction of speech and music. In some cases the speaker will have a frequency characteristic which can be used as complementary or supplementary to the frequency characteristic of the audio output. The arrangement of my invention lends itself to correct or modify the audio frequency characteristics of the amplifier. The selection of such characteristics is largely a question of personal choice. By adjusting the sum of capacities 46 and 47 as well as the ratio of 46 to the sum of 46 plus 41, the voltage feedback and its phase relation may be adjusted to a maximum at a desired frequency or band of frequencies.

By the way of example, the following values of resistances and capacities are cited, although I do not limit by invention to these particular values.

Resistances Capacities 18-1 megohm 17-0.000250 microfarads. 26-025 megohms 22-001 microfarads. 28-1 megohms 25-0000500 microfarads. 49-05 megohms. 32-0.005000 microfarads. 21-10000 ohms 43-8 microfarads. 45-500 ohms 44-8 microfarads.

46-0002 to 0.02 microfarads.

47-001 to 0.1 microfarads.

In the matter of cost, capacity 4'! is generally employed to by pass the screen grid resistor and is not strictly speaking an additional element. Capacity 46 may be from .002 to .02 microfarads. It is apparent that a condenser of 10 to 25 microfarads capacity is expensive; while one of .002 to .02 microfarads is relatively inexpensive. Practically it is only necessary to add the small feedback condenser 45 and, by virtue of my invention, the large expensive by pass condenser, usually employed across resistance 45, is omitted.

In Figure II I have illustrated my invention as applied to a screen grid detector tube and a pentode output tube; it is not necessary to limit the invention to these types of tubes. ample, ordinary triode tubes may be employed With a refiexed connection from the common resistance of the output tube to the input circuit of the precedingtube for the regeneration of audio frequency currents;

Such a reflexed arrangement is illustrated in Figure III. In this diagram 5I-is the primary input circuit. The secondary consists of inductance 52 and variable tuning capacity 53. One terminal of the tuned secondary circuit is connected through grid condenser '54 to the grid 55 of the tube 56. The grid condenser 55 is shunted by the grid leak 51. The cathode 58 is grounded at 59. The anode 66 is connected through resistance 6| to the B battery 62. The negative terminal of the B battery 62 is grounded at 63 and 62 is by-passed by condenser 64. The l secondary circuit may be grounded by a relatively high resistance 65. The value of resistance 65 may be of the order of several hundred thousand ohms.

The output of tube 56 is connected to tube 66 by connecting a coupling 'capacity 61 between anode 60 and control grid 168. The grid 68 is connected to ground by resistance ill. '.The anode 69 connects through the output device H to the B battery 62. through the self-bias resistance F4. ,Serially connected across 14 are two capacities 15 and 16. The junction of 15 and 16, is'connected to the common terminal of 52, 53; and 65; which is represented as 11. Cathode heating batteries are omitted for simplicity of illustration.

The operation of Figures II and III are substantially the same. In the case of Figure III the grid 55 serves a dual purpose; in one case, it acts as a radio input control; in the other case, it acts as the audio input control. The feedback voltage in Figure III is taken from the junction of 15-16 and feed back through- 52- and 51 to the grid 55.

While I have shown my invention as applied to the audio output of a relatively simple radio receiver, I do not wish my invention to be so limited. The method of off setting deleterious degenerative feedback by regenerative feedback which I have described may be. applied to audio The cathodel3; is grounded i For ex- I output tubes in tuned radio frequency receivers, superheterodynes, or any type of audio amplifier which employs circuits similar to Figure I.

I claim:

1. In an audio frequency amplifier, two vacuum tubes, each having an input circuit and an output circuit; the output circuit of the first of said tubes coupled to the input circuit of the second of said tubes; a resistance common to the input and output circuits of the last mentioned tube; two capacities in series shunting said resistance; and a connection from the junction of said capacities to an input circuit of the first mentioned tube.

2. In an audio frequency amplifier, two vacuum tubes, the first tube having two input circuits and one output circuit; the second tube having an input and an output circuit; the output circuit of the first of said tubes coupled to the input circuit of the second of said tubes; a resistance common to the input and output circuits of the last mentioned tube; two capacities in series shunting said resistance; and a connection from the junction of said capacities to one of the input circuits of the first mentioned tube.

3. An audio frequency amplifier comprising an input thermionic tube having grid, cathode, screen grid and plate electrodes; an output thermonic tube having grid, cathode and plate electrodes; an output circuit including the plate and cathode of the input tube; an input circuit including the grid and cathode of said output tube; a capacity coupling said input and output circuits; a second output circuit including the plate of said output tube and a loud speaker; a cathode biasing resistor common to the input and output circuits of said output tube; two capacities connected in series and shunting said common resistor; and a connection from the junction of said series capacities to the screen grid of said input tube.

4. An audio frequency amplifier comprising, an input vacuum tube having grid, cathode, screen grid and plate electrodes; an output tube having grid, cathode and plate electrodes; an output circuit including the plate and cathode of the input tube; an input circuit including the grid and cathode of said output tube; an ele ment coupling said input and output circuits; a second output circuit including the plate of said output tube and a loud speaker; a cathode biasing resistor common to the input and output circuits of said output tube; two capacities of unequal values connected in series and shunting said common resistor; and a connection from the junction of said series capacities to the screen grid of said input tube.

5. An audio frequency amplifier comprising, an input vacuum tube having grid, cathode, screen grid and plate electrodes; an output tube having grid, cathode and plate electrodes; an output circuit including the plate and cathode of the .input tube; an input circuit including the grid and cathode of said output tube; a capacity coupling said input and output circuits; a second output circuit including the plate of said output tube and a loud-speaker; a cathode biasing resistance common to the input and output circuits of said output tube; two capacities, whose impedance at low audio frequencies is substantially greater than the impedance of said common resistance, connected in series and shunting said common resistance; and a connection from the junction of said series capacities to the screen grid of said input tube.

6. An audio frequency amplifier comprising, an input vacuum tube having grid, cathode, screen grid and plate electrodes; an output tube having grid, cathode and plate electrodes; an output circuit including the plate and cathode of the input tube; an input circuit including the grid and cathode of said output tube; a capacity coupling said input and output circuits; a second output circuit including ,the plate of said output tube and a loud-speaker; a cathode biasing resistance common to the input and output circuits of said output tube; two capacities, whose reactance in ohms at low audio frequencies is greater than the value in ohms of the common resistance, connected in series and. shunting said common resistance; and a connection from the junction of said series capacities to the screen grid of said input tube.

CHARLES FREDERICK WOLL. 

